1. Technical Field
This invention relates to heat exchangers of the plate-fin type, particularly to large heat exchangers of this type adapted for use in ocean thermal energy conversion (OTEC) systems, and to plate-fin panels for use in such heat exchangers.
2. Background Art
Serious study of the possibilities of converting the potential energy represented by the difference in temperature between warm surface water and cold deep water in the ocean into useful form began at least fifty years ago with the researches of Georges Claude (see U.S. Pat. No. 2,006,985). Although the thermal energy available from ocean sources is essentially unlimited, the relatively low temperatures and small temperature differences involved result in very low plant thermal efficiencies, so that OTEC systems up to now have been uneconomic in comparison with fossil fuel plants. The dramatic increase in the cost of fossil fuels in recent years, however, has led in the cost of fossil fuels in recent years, however, has led to reconsideration of the economics of ocean thermal energy conversion.
Because of the small differences in temperature between the thermal source and thermal sink of an OTEC plant, and also because of the corrosive nature of, and marine organisms present in, the seawater medium, the effectiveness of the heat exchangers is a major factor in the efficiency and cost-effectiveness of an OTEC systems. Although conventional shell and tube exchangers have been proposed for OTEC plants, this type of exchanger presents serious drawbacks because of the difficulty of maintaining the seawater-side heat transfer surfaces free of fouling by algae and other marine organisms.
U.S. Pat. No. 4,055,145 issued on Oct. 25, 1977 to D. Mager and W. E. Heronemus and U.S. Pat. No. 4,062,189 issued on Dec. 13, 1977 to D. Mager, W. E. Heronemus, and P. M. J. Woodhead describe the use of plate-fin heat exchangers as evaporators and condensers for a closed-loop working fluid, such as ammonia, in an OTEC power generating plant. Based on an analytical study directed by the present inventor at the University of Massachusetts, and presented in a report entitled "Detailed Analytical Model of Rankine Cycle and Heat Exchangers for Ocean Thermal Difference Power Plants" under a grant, No. GI-34979, from the National Science Foundation, vertically arranged parallel plate-fin exchangers would permit maximum possible transfer of thermal energy between seawater flowing horizontally between spaced apart plate-fin units and working fluid flowing vertically within each unit. The above-mentioned U.S. Pat. No. 4,062,189, which is directed to a method of preventing the accumulation of micro-organisms in OTEC systems by alternating warm and cold seawater flow through the heat exchangers, also mentions that plate-fin heat exchangers are adapted for cleaning by brushing or scraping the flat plate-fin panel surfaces.
Further analytical and experimental studies have demonstrated the feasibility of the plate-fin heat exchanger concept presented in the above-mentioned Pat. Nos. 4,055,145 and 4,062,189; they have also demonstrated the necessity of having all surfaces exposed to seawater made of corrosion resistant material and the importance of maintaining these surfaces free of even minor amounts of biological fouling to avoid loss of heat transfer effectiveness. Copper-nickel alloys are well known for their resistance to corrosion by seawater, and also for their resistance to bio-fouling. Heat exchangers made exclusively of such alloys, however, would be very expensive, making an OTEC plant difficult to justify on an economic basis. In addition, these corrosion-resistant alloys have relatively low heat conductivities; so that the temperature drop across the heat exchanger surfaces can be a significant percentage of the available thermal difference in such a plant.
Among metals having a high heat conductivity, aluminum has long been used for evaporators in refrigerator freezing units and in automotive radiators, as well as for small heat exchangers in other types of service, because of its relatively low cost, light weight, and capability of being extruded into tubular elements of complex cross section, including multi-tubular members. Examples of such elements are shown in U.S. Pat. Nos. 2,190,494; 2,212,912; 2,415,243; 3,416,600; 3,486,489; 3,662,582; 3,668,757; 3,920,069; and 4,043,015.
Aluminum heat exchanger tubes are typically assembled in groups and attached to headers by soldering, brazing, welding, or use of adhesives (U.S. Pat. No. 3,416,600). Alternatively, or in addition, nonmetallic sealant layers, such as synthetic resins or natural or synthetic rubber may be used (U.S. Pat. Nos. 2,303,416; 2,385,542; 3,633,660; and 3,993,126).
Although aluminum has excellent heat transfer properties, it is easily corroded by seawater unless it can be suitably protected. Combining dissimilar metals to take advantage of respective characteristics such as high thermal conductivity and resistance to corrosion and bio-fouling is difficult, however, because of the danger of electrolytic corrosion if the different metals are exposed to seawater.
In addition to the problems of effective heat transfer, corrosion, and biological fouling which they share in common with other types of heat exchangers, plate-fin exchangers present unique structural problems. For effective thermal operation, the plate-fin panels should be thin and closely spaced, yet have a large surface area. This means that the plate-fin panels are very flexible, but intermediate supports can interfere with optimum flow of fluid past the exterior surfaces of the panels, as well as provide growth sites for bio-organisms. Also the flat thin panel configuration presents headering problems compared with conventional shell-and-tube exchangers, in which the tubes are rolled into the headers.